Retrospective analysis of efficacy and safety of oral paclitaxel for treatment of various cancers in dogs (2017–2021)

Abstract Background In humans, several safety evaluations have shown minimal adverse events with oral paclitaxel; however, its therapeutic efficacy and safety has not been well established in dogs with various cancers. Objectives We aimed to retrospectively evaluate the efficacy and safety of oral paclitaxel in dogs with various cancers. Methods Twenty‐one dogs diagnosed with various cancers were administered several doses of oral paclitaxel three times a month (group 1) or six times a month (group 2). Results The overall response rate was 6.25% (6.25%, complete response; 56.25%, stable disease; 37.5%, progressive disease) in dogs for which the treatment response could be evaluated. The median overall survival (OS) and progression‐free survival (PFS) were 74 and 60.5 days, respectively. Regardless of the administration group, differences in OS and PFS of the two groups did not reach statistical significance. Most dogs tolerated the treatment regimen well, and although minor adverse events were observed in some dogs, they recovered after temporary drug discontinuation, dose reduction or symptomatic treatment. There was no significant difference in the prevalence of adverse events between the two groups. Conclusions Based on the observed responses in certain types of cancers and the minimal adverse events, the study findings supported the efficacy and safety of oral paclitaxel administration in dogs. Thus, oral paclitaxel could play a role in the management of cancer in dogs.

been used alone and in combination with other drugs to treat various cancers, such as advanced ovarian cancer, metastatic breast cancer, non-small cell lung cancer, bladder cancer, and head and neck cancer (Hajek et al., 1996). In previous studies evaluating the safety of paclitaxel for various cancers, common adverse events included fatal hypersensitivity reactions, nausea, vomiting, loss of appetite and myelosuppression (neutropenia and thrombocytopenia) (Bocci et al., 2013;Lang et al., 2013;Rowinsky et al., 1989). Among these adverse events, hypersensitivity reactions were caused by the action of Cremophor ® EL (BASF Corp., Ludwigshafen, Germany) added to the treatment regimen to solubilise paclitaxel (Nehate et al., 2014;Picard & Castells, 2015).
The use of paclitaxel has not been frequently described in veterinary medicine owing to the high prevalence of acute hypersensitivity reactions to conventional Cremophor ® EL-added drugs. In a study of dogs treated for cancer with paclitaxel at a dose of 165 mg/m 2 through slow IV infusion every 3 weeks, hypersensitivity reactions were frequent (64%) despite pre-treatment with corticosteroids (Poirier et al., 2004).
Although a low dose of 132 mg/m 2 has been suggested for minimising adverse events (Poirier et al., 2004), the high prevalence of hypersensitivity reactions has made paclitaxel with conventional Cremophor ® EL administration to veterinary cancer patients challenging. To overcome this difficulty, substances that do not cause hypersensitivity reactions when added to paclitaxel have been developed for safe drug delivery.
Novel paclitaxel formulations have been designed to increase water solubility and reduce the risk of hypersensitivity reactions using non-Cremophor ® formulations (Khanna et al., 2015).
The newly developed injectable paclitaxel has the advantage of safe delivery to cancer patients without hypersensitivity reactions and has been recommended as a therapeutic agent for various cancers in human patients (Miele et al., 2009;Stinchcombe, 2007). In contrast to several clinical trials in human medicine investigating treatment with new formulations of paclitaxel (Miele et al., 2009;Stinchcombe, 2007), only a few studies exist in the field of veterinary medicine. A watersoluble micellar paclitaxel (Paccal Vet) formulation showed an effect on canine haemangiosarcoma in vitro (Reckelhoff et al., 2019). Subcutaneous administration of non-Cremophor ® paclitaxel to dogs diagnosed with cancers showed anti-cancer effects similar to those of conventional paclitaxel (Taxol ® ) and a low prevalence of hypersensitivity reactions (Selting et al., 2018;Silva et al., 2015).
Among the newly developed formulations of paclitaxel, the paclitaxel developed for oral intake has the advantage of easy administration as well as a longer exposure in human patients with various cancers, which is an important factor in the efficacy of cell-cycle phasespecific agents such as paclitaxel (Kang et al., 2018;Ryu et al., 2017).
In a previous study conducted in human patients with advanced gastric cancer, the efficacy of oral paclitaxel was similar to that of conventional chemotherapy drugs, with a notably low prevalence of hypersensitivity reactions (Kang et al., 2018).
Although oral administration of paclitaxel to dogs with bladder cancer in a previous study conducted in our lab suggested the possibility of safe administration with minimal adverse effects (Chae et al., 2020), no study exists on the long-term efficacy and safety of oral paclitaxel treatment on dogs with various cancers. Therefore, we aimed to ret-rospectively evaluate the efficacy and safety of oral paclitaxel in dogs with various cancers.

Patient selection
This study was designed to evaluate the efficacy and safety of oral

Treatment protocol
Owing to the lack of studies on the protocol for oral paclitaxel dosing in veterinary medicine, the therapy was started with a low dose, which was adjusted according to each dog's response and condition. The dosing schedule was established by referring to the clinical trial results of oral paclitaxel in patients with various cancers in human medicine and various research studies on experimental animals (Chae et al., 2020;Hahn et al., 2014;Kang et al., 2018;Pallis et al., 2008). Dogs were administered oral paclitaxel three times a month (group 1) or six times a month (group 2). The dogs in group 1 received paclitaxel orally on days 1, 8, and 15 over 4 weeks. Dogs in group 2 were administered the drug on days 1, 4, 8, 11, 15, and 18 over 4 weeks. For a greater anti-cancer effect, group movement was performed in some dogs ( Figure 1).
During the treatment, drug-related adverse events were evaluated by a veterinarian at each visit for the determination of the next dosing regimen. When no major side effects were observed at the initial dose, the dose was gradually increased until tolerated, as evaluated according to the absence of adverse events such as bone marrow suppression. The final dose was determined according to cancer progression and the patient's condition. The maximum weekly dose was set not to exceed 30 mg/kg, referring to the no-observed-adverse-effect-level of 5 mg/kg/day or less in the 6-week repeated-dose toxicity study in beagle dogs conducted by a pharmaceutical company (unpublished data).

F I G U R E 1 Flow diagram of changes in group composition of dogs enrolled in this study
Treatment was continued until confirmation of significant disease progression, withdrawal of the owner's consent, or unacceptable toxicity, such as life-threatening adverse events, were identified.

Efficacy and safety evaluation
To evaluate the efficacy of oral paclitaxel, imaging tests were per- On long-term administration of the drug >8 weeks in dogs, evaluation of liver/kidney dysfunction was conducted through regular blood analysis. Fisher's exact test was used to compare the difference in the incidence of adverse events between the groups. Statistical significance was set at p < 0.05.

Patient characteristics
Twenty-one dogs who received oral paclitaxel were included in the analysis. Of the 21 dogs, 2 were undergoing treatment at the time of the study, and the remaining died or discontinued follow-up. Cancer diagnosis of enrolled dogs was made by cytology (n = 13) or biopsy (n = 5) or both (n = 3). In 16 of 21 dogs, oral paclitaxel treatment was started immediately without any other treatment within one month after diagnosis. The remaining five dogs were converted to oral paclitaxel after preceding treatment with surgery (n = 1), chemotherapy (n = 2; vinblastine for TCC treatment, both cases) or metronomic chemotherapy (n = 2; cyclophosphamide and toceranib phosphate, respectively). The dogs included 7 castrated males, 11 spayed females and 3 intact females. The patient's age ranged from 5 to 16 years Cocker Spaniel (n = 2), Bichon Frise (n = 1) and Golden Retriever (n = 1).

Treatment results
Dogs Initially, 12 dogs were in group 1 and 9 dogs were in group 2. After evaluating the occurrence of adverse events among the dogs in group 1, some dogs were moved to group 2 for the purpose of increasing anticancer efficiency through dense dose delivery. In the final analysis, 7 dogs were in group 1, and 14 dogs were in group 2 (Figure 1). The median dose of oral paclitaxel delivered was 10 mg/kg per week (range: 3-30 mg/kg) in both groups.
The treatment was discontinued due to a progressive course of disease (n = 9), loss of follow-up (n = 5), occurrence of neutropenia (n = 2) and death (n = 3) in 19 of 21 dogs at the time of investigation.
Tumour size assessment following initiation of treatment in 16 dogs could be evaluated by RECIST v 1.1. The ORR was 6.25% in dogs capable of evaluating anti-cancer response through RECIST. One dog achieved CR and 9 dogs (56.25%) achieved SD. The dog with pulmonary carcinoma achieved CR when confirmed by CT about 2 months after the initial administration of oral paclitaxel, and CR was maintained for more than 553 days until the time of investigation (Figure 2a-d). Nine dogs achieved SD, and their diagnoses were TCC (n = 4), oral SCC (n = 1), mammary gland carcinoma (n = 2), lymphoma with pulmonary involvement (n = 1) and pulmonary carcinoma (n = 1).

Adverse events
Most dogs tolerated oral paclitaxel well. The adverse events observed were bone marrow suppression (n = 3), vomiting (n = 2) and diarrhoea (n = 3) (Table 2). However, they were mostly mild (VCOG grade 1-2), and clinical signs resolved following drug dose reduction (n = 3), drug discontinuation (n = 3) and symptomatic treatment (n = 2). Neither the drug delay duration nor the symptomatic treatment duration was long (duration, <1 week). In the case of dose reduction, the dose was reduced to the dose prior to gradual escalation owing to the absence of a recommended dosing regimen and protocol in dogs. There was no significant difference in the occurrence of adverse events according to the paclitaxel administration group (p = 0.346).

DISCUSSION
This study was intended to evaluate the safety and efficacy of oral paclitaxel in dogs with naturally occurring cancers. Although proving the efficacy of oral paclitaxel in this study was difficult, it was confirmed that it is a relatively safe drug without life-threating hypersensitivity as that seen with conventional Taxol.
F I G U R E 2 (a) Transverse and (b) dorsal plane computed tomography (CT) images of a dog with a relatively large mass of soft tissue that appeared as attenuation at the region of accessory lung lobe with a small, central air bronchogram (arrow) and multi-focal small nodules of soft tissue attenuation (arrowheads). Primary lung tumour was diagnosed as pulmonary carcinoma by ultrasound-guided fine needle aspiration. (c) Transverse and (d) dorsal plane CT images of the same dog after 10 months of oral paclitaxel administration. The previous attenuation at the accessory lung lobe was no longer visible (arrow) and small lung nodules in the lung field disappeared The conventional formulation of paclitaxel is an IV injection product solubilised in Cremophor EL ® and ethanol (Nehate et al., 2014;Picard & Castells, 2015). Cremophor EL ® and ethanol can solubilise the poorly soluble paclitaxel and effectively deliver the drug to cancer cells to achieve anti-cancer effects. However, they can cause strong hypersensitivity reactions due to their toxicity. Therefore, conventional paclitaxel requires pre-treatment with steroids and anti-histamines, and should be slowly infused for more than 3 h with serial monitoring.
These drawbacks limited the use of conventional paclitaxel in dogs and cats with cancers (Kim et al., 2015;Poirier et al., 2004). Recently, a new paclitaxel formulation was developed for safe and efficient drug delivery using a non-Cremophor EL ® formulation (Khanna et al., 2015;Nehate et al., 2014). The oral paclitaxel used in our study was solubilised using a mixture of monoolein, tricaprylin and polysorbate 80, which had a better safety profile compared to conventional formulation (Jang et al., 2017). The mucosal adhesive properties of this new formulation improve the permeability and contribute to the effective distribution of the drug in major organs after absorption in the gastrointestinal tract (Hong et al., 2007;Jang et al., 2017).
In terms of efficacy, it was difficult to demonstrate the efficacy of oral paclitaxel with statistical significance due to the lack of enrolled dogs and the characteristics of the retrospective study. Nevertheless, the results of our study suggest the potential use of oral paclitaxel in the treatment of dogs with TCC and pulmonary carcinoma. Determining the clear effect of paclitaxel in TCC was challenging due to the small number of cases and co-administration with NSAIDs; however, our findings are similar to those reported in a previous oral paclitaxel case report (Chae et al., 2020) that suggested the possibility of using TA B L E 1 Median overall survival (OS), progression-free survival (PFS) according to group 1 and 2 and p value and hazard ratio between groups with pulmonary carcinoma, CR was observed in one case and SD was noted for more than 497 days in another case. Although further studies are needed, these two cases support the possibility of effective and safe administration of oral paclitaxel for dogs with pulmonary carcinoma, especially since paclitaxel has been used for non-small lung cancer in humans (Akerley III, 2000;Hajek et al., 1996). However, many dogs enrolled in this study exhibited PD despite oral paclitaxel administration. This low treatment response may be attributed to lack of a therapeutic effect of this drug on the other types of cancers; however, it could also be associated with the difficulty in distinguishing the treatment effect because some dogs already had cancers in a terminal state.
In fact, 6 of the enrolled dogs were treated with oral paclitaxel in an already metastatic state. Oral paclitaxel was preferred in these metastasised dogs owing to convenient dosing and fewer hospital visits; however, it is thought that this study may have resulted in a low response rate. In addition, this low response rate could be attributed to the inclu-sion of non-epithelial-derived cancer cases in this study, although taxanes are proven therapeutic agents chiefly for epithelial-derived cancer in human medicine (Maloney et al., 2020).
For the safety evaluation, adverse events were graded retrospectively and oral paclitaxel was noted to be a safe drug with only minor adverse events. Gradual dose changes were performed in this study owing to the lack of an established safe therapeutic dose for maximal efficacy in dogs with naturally occurring cancers. The observed adverse events were gastrointestinal symptoms and bone marrow suppression. Of the three cases of bone marrow suppression, two cases were VCOG grade 1 and 1 case was VCOG grade 2. Of the five cases of gastrointestinal symptoms, only one case of diarrhoea was VCOG grade 2, and all the others had mild symptoms of VCOG grade 1.
All symptoms resolved after temporary drug discontinuation, dose reduction or symptomatic treatment such as anti-diarrheal drugs, and fluid therapy. In addition, no liver or kidney toxicity associated with drug accumulation was demonstrated in dogs administered long-term paclitaxel for 8 weeks or more. In contrast to the high rate of hypersensitivity reactions that occurred when conventional Taxol was administered to dogs (Kim et al., 2015;Poirier et al., 2004), no hypersensitivity reactions were observed with oral paclitaxel. The safety of oral paclitaxel administration without pre-treatment signifies that it can be considered as a treatment for dogs diagnosed with various cancers.
In addition to demonstrating the safety of the drug and suggesting its potential as a therapeutic agent in certain cancers, this study presents an appropriate drug-delivery protocol for dogs diagnosed with cancer owing to the lack of well-established protocols in small animals. A delivery schedule of days 1, 8 and 15 every 4 weeks was applied to patients in group 1 based on the results of a previous study by Kang et al. (2018).

CONCLUSION
Although the efficacy of the drug in various cancers has not been demonstrated, this study supports the possibility that oral paclitaxel could be used as an effective drug for TCC and pulmonary carcinoma. In addition, it was safely administered with minimal toxicity to dogs compared to IV injection. Further studies on specific cancers and stages in companion animals should be conducted to demonstrate the potential of oral paclitaxel in the effective treatment of animals diagnosed with various cancers.